The invention is concerned with a controller for a linear accelerator, particularly but not exclusively for use in an ion implanter.
A linear accelerator structure accelerates charged particles of a specific mass/charge ratio which are injected into the accelerator at a specific injection energy Radio frequency (rf) linear accelerators have been known for many years from the field of nuclear physics where they have been employed to accelerate heavy ions. More recently, rf accelerators have been used in semiconductor wafer processing. Typically, a beam of ions of a required species (such as boron, phosphorous, arsenic or antimony) is produced and directed at a wafer so that the ions become implanted under the surface of that wafer. Although electrostatic acceleration systems are suitable for producing beams of singly charged ions of 200 keV or more, it has been recognised that the desirable characteristics (for certain applications) of relatively high beam current and relatively high beam energy can be achieved by including an rf accelerator in the ion implanter device.
The use of rf linear accelerators for implantation of ions into semiconductor wafers has been suggested at least since 1976 in xe2x80x9cUpgrading of Single Stage Acceleratorsxe2x80x9d by K. Bethge et al, pages 461-468, Proceedings of the Fourth Conference on the Scientific and Industrial Applications of Small Accelerators, North Texas State University, Oct. 27-29, 1976; and in xe2x80x9cHeavy Ion Post-acceleration on the Heidelberg MP Tandem Acceleratorxe2x80x9d, edited by J. P. Wurm, Max Planck Institute for Nuclear Physics, Heidelberg, May 1976. U.S. Pat. No. 4,667,111 discloses an ion implanter incorporating a radio frequency linear accelerator to provide ultimate beam energies as high as 2 MeV or more.
As discussed by Glavish et al in xe2x80x9cProduction of high energy ion implanters using radio frequency accelerationxe2x80x9d, Nuclear Instruments and Methods in Physics Research B21 (1987), at pages 264 to 269, it is necessary that each resonator in the rf accelerator be kept in precise tune and matched to its amplifier, for example by feedback control of a movable plate capacitor. The resonators tend to be sensitive to thermal and mechanical disturbances as they are part of highly tuned systems, with Q values between 1000 and 2000. It is also important that the amplitude and phase of the rf voltage at the acceleration electrode be controlled. In one arrangement, a signal from the inductive or capacitative probes associated with each cavity is compared with the desired phase and amplitude derived from a master oscillator via a precision phase shifter. Using a microprocessor, a xe2x80x9cparameter setxe2x80x9d for a given ion beam energy and species may be developed. Phase may be held to about 1xc2x0 and amplitude to within 1%.
U.S. Pat. No. 5,801,488 also describes the control of an rf accelerating device. Here, a control unit determines the respective values of phase and rf power, based upon a predetermined programmed algorithm, to obtain a target energy which is set by an operator. The controller adjusts the phase and amplitude under feedback control. In xe2x80x9cThe Development of a Beam Line using an RFQ and 3-Gap RF Accelerators for High Energy Ion Implantersxe2x80x9d, presented by Fujisawa et al at IIT in Kyoto, Japan, Jun. 24 1998, a personal computer is employed to control phase and amplitude to an RFQ and 3-gap rf beam line. Again, phase is controlled to around 1xc2x0 and amplitude to around 0.5%.
It will thus be appreciated by those skilled in the art that the precision and stability of the system relies upon the ability to generate a signal, for each resonator, which has a precise phase and amplitude. It is also important that the relative phase shift between resonators is accurately maintained.
One object of the present invention is accordingly to stabilize and set the phase shift between signals as it fluctuates due to mechanical or thermal drift, for example. It is a further object to provide a technique for introducing an accurate chosen phase shift into a sinusoidal signal. Still a further object is to accurately determine and control the amplitude of such a signal.
In a first aspect of the invention, here is provided a controller for controlling a phase shift between a reference signal and a measured signal in an rf resonator having an rf power supply, the controller comprising an oscillator for providing a reference sinusoidal signal having a reference phase; a detector for generating a transduced signal from the rf resonator, the transduced signal having a detected phase; a phase shifter apparatus including a quadrature signal generator arranged to shift the phase of the reference sinusoidal signal by 90xc2x0 relative to the reference phase so as to generate a reference cosinusoidal signal; a phase demand signal generator arranged to generate a first phase demand signal representing the sine of a desired angle of phase shift of the said reference sinusoidal signal plus a further 90xc2x0 phase shift, and to generate a second phase demand signal representing the cosine of the said desired angle of phase shift plus a further 90xc2x0 phase shift; a first multiplier arranged to multiply the said cosinusoidal reference signal with the first phase demand signal representing the sine of desired angle of phase shift plus 90xc2x0 to generate a first composite signal, and a second multiplier to multiply the said sinusoidal reference signal with the second phase demand signal representing the cosine of the desired angle of phase shift plus 90xc2x0 generate a second composite signal; and a summer arranged to sum the first and second composite signals to generate a phase shifter output signal which is a second sinusoidal signal that is shifted in phase relative to the reference phase of the reference sinusoidal signal by the said desired angle of phase shift plus 90xc2x0, the second sinusoidal signal being equivalent to a second cosinusoidal signal that is shifted in phase relative to the reference phase of the reference cosinusoidal signal by the said desired angle of phase shift; a second multiplier arranged to multiply the transduced signal with the second cosinusoidal signal and to generate a phase error signal having a dc component from the resultant product; and a processor arranged to generate a control signal on the basis of the dc component of the phase error signal, to control the output of the said power supply so as to minimize the dc phase error signal.
The controller of the present invention relies upon the trigonometrical identity
sin(xcfx89t+b)=sin(xcfx89t)cos(b)+cos(xcfx89t)sin(b) 
where the phase shift in the sinusoidal signal is represented by xe2x80x9cbxe2x80x9d.
Sin(b) and cos(b) are dc values which may be accurately generated. Thus, precise linear adjustment of the phase shift relative to a master oscillator may be provided. The phase angle xe2x80x9cbxe2x80x9d may be continuously adjusted over a full 360xc2x0 and with no discontinuity. The linearity and stability of the apparatus is also improved relative to the prior art.
It is desirable to ensure that the phase of the second sinusoidal signal (having a xe2x80x9cdemand phasexe2x80x9d accurately determined using the trigonometrical function outlined above) is identical with the phase of the rf signal in the rf resonator which is obtained by the detector. When this is the case, the product of the second sinusoidal signal, shifted by exactly 90xc2x0, (so that it becomes the second cosinusoidal signal) and the transduced signal, should be zero. This principle can be used to provide a phase controller which uses the accurately determined phase shift as a reference to which the phase of the rf signal in the resonator cavity is locked via closed loop feedback. With this technique, phase can be controlled to about 0.5xc2x0. It will be appreciated that, instead of shifting the desired phase angle by 90xc2x0 so as to produce, in effect, the second cosinusoidal signal, a quadrature signal of the transduced signal may instead be multiplied by a sinusoidal signal shifted by the chosen phase shift only (that is, not by an additional 90xc2x0) to create a phase error signal having a dc component. Alternatively, this sinusoidal signal (phase shifted by the desired angle of phase shift only) can be generated and then passed through a second quadrature signal generator which converts it, in effect, into the second cosinusoidal signal.
The quadrature signal generator may, for example, be an accurate single delay cable. Although this is adequate for a fixed frequency apparatus, a stripline structure is preferred for variable frequency devices. For example, the stripline structure, provided with taps and jumpers, can be embedded into a circuit board. This potentially allows sub-nanosecond adjustment of the time delay provided by the sub-nanosecond structure, such that a precise 90xc2x0 phase shift can be made to the first sinusoidal signal for a range of signal frequencies.
The multiplier is in preference a fast analogue multiplier, such as the high precision AD834 or AD835 multiplier manufactured by Analog Devices. In that case, the d.c. values of sin(b+90) and cos(b+90) may be generated by a digital to analog converter (DAC). In preference, a pair of 16 bit DAC""s are employed, operating under microprocessor control.
The controller of the invention is particularly suitable for application to a resonator which is part of an rf accelerator. Specifically, it may be desirable to apply a signal of a first known relative phase to a first resonator, and to apply a signal of a second known relative phase to a second resonator which Is, for example, downstream of the first resonator This may be done using a single apparatus controller arranged to generate two separate phase shifts relative to a common signal having a reference phase, or by using two separate apparatuses (again preferably relying upon a common signal having a reference phase). It may desirable that the first and the second relative phases are equal, that is, there is no phase difference between the signal applied to the first and the signal applied to the second resonator.
The controller may further comprise scaling means for attenuating the amplitude of the reference sinusoidal signal by a predetermined fraction to generate a scaled reference sinusoidal signal having a predetermined amplitude. Likewise, the scaling means may be further arranged to attenuate the amplitude of the reference cosinusoidal signal by the predetermined fraction to generate scaled reference cosinusoidal signal having the said predetermined amplitude.
The scaling means may be further arranged to attenuate the amplitude of the transduced signal by the said predetermined fraction.
In one embodiment, the scaling means may be arranged to attenuate the demand signal generated by the processor to a predetermined fixed amplitude.
The controller may further comprise signal processor means arranged to control the amplitude of the rf signal in the rf resonator, the signal processor means being configured to receive the said transduced signal from the detector, and to calculate an amplitude error signal by comparing the amplitude of the said transduced signal with a reference signal having a reference amplitude; the controller being further arranged to adjust the amplitude of the control signal generated by the processor in dependence upon the said amplitude error signal so as to minimize the amplitude error signal.
The analog multipliers of preferred embodiments have a maximum input voltage of 1.25 V peak and scaling the signals is therefore desirable. Furthermore, scaling the signals to a reference voltage eliminates the effects of non-linearities which arise in amplitude detection circuitry.
In a further aspect of the invention, there is provided a controller for controlling a phase shift between a reference signal and a measured signal in an rf resonator having an rf power supply, the controller comprising an oscillator for providing a reference sinusoidal signal having a reference phase; a detector For generating a transduced signal from the rf resonator, the transduced signal having a detected phase; a phase shifter apparatus including a quadrature signal generator arranged to shift the phase of the reference sinusoidal signal by 90xc2x0 relative to the reference phase so as to generate a reference cosinusoidal signal; a phase demand signal generator arranged to generate a first chase demand signal representing the sine of a desired angle of phase shift of the said reference sinusoidal signal, and to generate a second phase demand signal representing the cosine of the said desired angle of phase shift; a first multiplier arranged to multiply the said cosinusoidal reference signal with the second phase demand signal representing the cosine of the desired angle of phase shift to generate a first composite signal, and to multiply the said sinusoidal reference signal with the first phase demand signal representing the sine of the desired angle of phase shift to generate a second composite signal; a summer arranged to generate a phase shifter output signal by determining the difference between the said first and said second composite signals, the phase shifter output signal being a second cosinusoidal signal which is shifted in phase relative to the reference phase of the reference cosinusoidal signal by the said desired angle of phase shift; and a second multiplier arranged to multiply the transduced signal with the second cosinusoidal signal and to generate a phase error signal having a dc component from the resultant product; and a processor arranged to generate a demand signal on the basis of the dc component of the phase error signal, to control the output of the said rf power supply so as to minimize the phase error signal.
Here, the trigonometrical relationship
cos(xcfx89t+b)=sin(xcfx89t)sin(b)xe2x88x92cos(xcfx89t)cos(b) 
is employed, so that the resultant phase shifted signal is cosinusoidal.
In further aspects of the invention, methods of controlling a phase shift between a reference signal and a measured signal are provided.
In still a further aspect of the present invention, there is provided an apparatus for measuring the amplitude of an rf signal in an rf resonator having an rf power supply, comprising a signal processor means configured to receive as a first input, a transduced signal representative of the amplitude of the rf signal, and to receive, as a second input, a command scaling signal having a predetermined amplitude, the signal processor means being arranged to generate a scaled transduced signal having an amplitude scaled by an amount directly proportional to the predetermined command scaling signal amplitude; means for generating a reference signal having a reference amplitude; a comparator arranged to compare the amplitude of the scaled transducer signal with the reference amplitude of the reference signal and to generate an amplitude error signal representative of the difference between the scaled transducer signal amplitude and the reference signal amplitude; the signal processor means being further arranged to adjust the output of the rf power supply in dependence upon the amplitude error signal so as to minimize the subsequent difference between the scaled transducer signal amplitude and the reference signal amplitude.
By scaling the transduced signal rather than trying to measure the amplitude directly, the inaccuracy arising from the non-linearities present in peak measurement devices is eliminated.
A fast analog multiplier may be used to carry out scaling. One of the multiplicands is the transduced signal to be scaled, and the other is a variable analog signal generated, for example, by a DAC. Suitably, pre-scaling by a fixed fraction is also carried out, for example by using a network of resistors.
The invention also extends to a phase shifter apparatus for generating a phase shift in a sinusoidal signal, comprising: an oscillator for generating a first sinusoidal signal having a reference phase; a quadrature signal generator arranged to shift the phase of the said sinusoidal signal by 90xc2x0 relative to the said reference phase so as to generate a first cosinusoidal signal; a desired phase shift signal generator arranged to generate a first phase signal representing the sine of a desired angle of phase shift of the said first sinusoidal signal, and a second phase signal representing the cosine of the said desired angle of chase shift; a first multiplier arranged to multiply the said cosinusoidal signal with the first phase signal representing the sine of the said desired angle of phase shift, to generate a first composite signal, and a second multiplier to multiply the said sinusoidal signal with the second phase signal representative of the cosine of the said desired angle of phase shift, to generate a second composite signal; and a summer arranged to sum the first and second composite signals to generate a phase shifter output signal which is a second sinusoidal signal that is shifted in phase relative to the reference phase of the first sinusoidal signal by the said desired angle of phase shift.
In still a further aspect of the present invention there is provided a phase shifter apparatus for generating a phase shift in a cosinusoidal signal comprising: an oscillator for generating a first sinusoidal signal having a reference phase; a quadrature signal generator arranged to shift the phase of the said sinusoidal signal by 90xc2x0 relative to the said reference phase so as to generate a first cosinusoidal signal; a desired phase shift signal generator arranged to generate a first phase signal representing the sine of a desired angle of phase shift of the said first sinusoidal signal, and a second phase signal representing the cosine of the said desired angle of phase shift; a first multiplier arranged to multiply the said first cosinusoidal signal with the second phase signal representing the cosine of the said desired angle of phase shift to generate a first composite signal, and a second multiplier to multiply the said first sinusoidal signal with the said first phase signal representing the sine of the said desired angle of phase shift to generate a second composite signal; and a summer arranged to generate an output representative of the difference between the said first and second composite signals, which output is a second cosinusoidal signal that is shifted in phase by the said desired angle of phase shift relative to the phase of the first cosinusoidal signal
Methods of generating a phase shift in a sinusoidal signal and in a cosinusoidal signal are also provided by the invention.
The invention also extends to an rf accelerator including a controller incorporating the invention as defined in the claims, and to an ion implanter for implanting ions into a substrate employing such an rf accelerator.
There follows by way of example only a description of a preferred embodiment of the invention.